Speculation is mounting over the identity of two people whose affair has reportedly left David Cameron ‘stunned’.

Although the names of the middle-aged pair involved or the nature of the affair cannot be revealed for legal reasons, it does not involve anyone in the current cabinet.

It is understood that before the Mail on Sunday splash Mr Cameron was told of the relationship, which has now come to an end.

According to sources, the prime minister ‘immediately realised the importance of the story’ and spoke to his aides about the possible fallout if details went public.

They also discussed how such disclosure could ‘blow out of the water’ any major political set pieces planned by Downing Street.

One senior source said: ‘This revelation is dynamite. None of us could believe it when we first heard it. Then we just thought, “What a complete mess”.’

The news had caused ‘great personal distress to innocent parties’, the source added. It was reported David Cameron held crisis talks over the affair though a Downing Street spokesman later denied such a meeting had taken place.

Labour MP Chris Bryant tweeted if their names are revealed it will ‘sink’ the prime minister.

Other Twitter users posted clues as to the identity of the couple who had the affair, though some suggested it was Larry and Freya, the Downing Street cats.